The invention concerns a molding process for making a pneumatic tire having a wide configuration and a wide surface for rolling, that is a wide tread for the portion of the tire which touches the ground.
The tire is particularly well suited for all types of ground surfaces, particularly uneven road beds. When the tire is inflated to a low pressure, it is wider that it is high and has a profile that is more or less eliptical in cross section. It is known to use tires having a wide road contact surface which are inflated at a low pressure to carry a load on grounds which are stony, sandy or otherwise generally not made into roads. Because these tires are fabricated wider than they are high, they have less tendency to get stuck in the sand or to be arrested by various articles. Such tires when cut transverse and viewed in cross section have a profile that is more or less eliptical. Such tires are advantageously manufactured from synthetic plastic material such as polyvinylchloride, which is hot molded in accordance with known processes of the thermal plastic industry. The most common process for obtaining such tires comprises molding by rotation. The material to be molded is introduced into a mold and the mold is rotated and heated in order to obtain a spreading or a placing of the plastic material on the internal walls of the mold and a curing or gelification, that is, hardening of the plastic molded material. The term gelification or curing is used more particularly with certain plastic materials, particularly polyvinylchloride when it passes from the liquid solution state to the solid state under the action of heat.
In such a process, the plastic material has a tendency to spread rather uniformly on the internal walls of the mold while the mold is rotating. Experience has shown that the tires so obtained have weaknesses when under load, particularly when these tires have their extremities, which have their smallest diameter, traversed by a carrying axis. The quantity of plastic material at the level of the axis of the wheel and at each extremity of this axis is too weak to resist the load of the carried weight. Additionally, the strain due to the running of the wheel causes frequent breakdown of the tire, particularly between the extremities of the axis of the tire and the road contacting surface which is related thereto.
To correct these shortcomings, reinforcing the extremities of the tires close to the carrying axis by various means has been attempted such as by employing the assistance of supports such as, for example, containers which partially hold the casing. These means are difficult and complicated to put into effect, they cause a heterogenious or nonhomogenious tire structure and the cost of manufacture is considerably increased.
In U.S. Pat. No. 3,470,933 to Molnar, a method of molding an entire tire by rotational molding is disclosed. However, the use of preformed reinforced extremities is not taught or suggested.